Polymers are often used in producing preforms (parisons) which are heated with infrared heating lamps prior to being blow-moulded into articles, including liquid containers such as beverage bottles and the like. The heat lamps used for reheating polymer preforms (parisons) for the commercial manufacture of liquid containers such as beverage bottles are typically quartz lamps having a broad light emission spectrum from 500 nm to greater than 1500 nm, i.e. infrared heating lamps. The maximum light emission from these lamps occurs typically in the range of about 1100 to 1300 nm. Polyester, especially polyethylene terephthalate (“PET”), absorbs poorly in the region between 500 to 1400 nm. Thus, in order to speed up the reheat step in bottle production, or to reduce the amount of energy required for reheat, agents which absorb light in the region between 700 to 1300 nm can be added to the polyester polymer as reheat additives.
A variety of black and grey body absorbing compounds have previously been used as reheat agents to improve the range of heating characteristics of polyester under infrared heating lamps. These compounds are typically black iron oxide, elemental antimony, black carbon and copper chromite. The term “black carbon” includes graphite, any form of carbon black, charcoal, activated carbon and the like. However, these materials all have greater intrinsic absorptivity in the visible spectrum between 400 and 700 nm than in the infrared region between 700 and 1400 nm. This makes these materials appear inefficient when the visible impact versus infrared absorptivity is considered. High levels of reheat cannot be achieved without the severe darkening of the polymer. While the impact of these materials on the visual appearance of preforms can be reduced by using relatively large particles of the additives, they inherently cannot exhibit greater absorptivity in the infrared region of the electromagnetic spectrum relative to the visible region of the spectrum. Therefore, the amount of absorbing materials that can be added to a polymer is limited by the impact of those materials on polymer visual properties such as transparency. This is particularly pertinent if the preforms are to be used to manufacture liquid containers such as beverage bottles, especially for use in containing mineral water, where high transparency and an absence of colour are considered essential. Transparency is usually represented as “L*” in the CIELAB system, with 100 being the highest and 0 being the darkest. Generally, darker coloured reheat agents can be added in only very small quantities because of their negative impact on L*.
WO2005/095516 (Colormatrix) discloses a number of inorganic additives that intrinsically possess greater absorption in the near infrared region relative to absorption in the visible spectrum. Additives disclosed include titanium nitride, lanthanum boride, and indium tin oxide (ITO).
US2006/016146 (Eastman) discloses polyester polymer and copolymer compositions containing titanium nitride particles.
However, there are still limitations with the materials described in the prior art. More particularly, it is found that preforms which include titanium nitride as described in the aforementioned patent applications have reducing absorbances as the wavelength increases from 700 nm. The more the absorbance decreases as the wavelength increases from 700 nm, then the more titanium nitride will be required to be included in a preform to achieve a desired level of reheat. However, increasing the level of titanium nitride will detrimentally reduce the clarity and L* of the preform.
It is an object of the present invention to address problems associated with the reheat characteristics of polymer compositions.